CK-12 Biology I - Honors

by CK-12 Foundation

  The Heart

  The heart is the muscular organ that pumps blood through the blood vessels by repeated, rhythmic contractions. The term cardiac means "related to the heart" and comes from the Greek word kardia, for "heart." The heart is made up mostly of cardiac muscle tissue, (shown in Figure below) which contracts to pump blood around the body. In adults, the normal mass of the heart is 250-350 grams (9-12 oz), or about three quarters the size of a clenched fist, but badly diseased hearts can be up to 1000 g (2 lb) in mass due to enlargement of the cardiac muscle. For an animation of the heart's anatomy, see http://www.byrnehealthcare.com/animations/SutterAnatomy.htm.

  Figure 22.2

  External and internal views of the human heart. The aorta in the photo cannot be seen clearly because it is covered by a layer of adipose tissue (fat).

  The heart is usually found in the left to middle of the chest with the largest part of the heart slightly to the left. The heart is usually felt to be on the left side because the left ventricle is stronger (it pumps to all the body parts). The heart is surrounded by the lungs. The left lung is smaller than the right lung because the heart takes up more room in the left side of the chest. The position of the heart within the chest is shown in Figure below.

  Figure 22.3

  Position of the heart in relation to the lungs. The heart can be seen in the lower middle area of the figure, behind the lungs.

  Blood Flow Through the Heart

  Blood flows through the heart in two separate loops; you could think of them as a “left side loop” and a “right side loop.” The right side and left side of the heart refer to your heart as it sits within your chest. Its left side is your left side and, its right side is your right side.

  The right side of the heart collects deoxygenated blood from the body and pumps it into the lungs where it releases carbon dioxide and picks up oxygen. The left-side carries the oxygenated blood back from the lungs, into the left side of the heart which then pumps the oxygenated blood throughout the rest of the body.

  The heart has four chambers, the two upper atria and the two lower ventricles. Atria (singular, atrium) are the thin-walled blood collection chambers of the heart. Atria pump the blood into the ventricles. Ventricles are the heart chambers which collect blood from the atria and pump it out of the heart. On the right side of the heart, deoxygenated blood from the body enters the right atrium from the superior vena cava and the inferior vena cava, shown in Figure below. Blood enters the right ventricle which then pumps the blood through the pulmonary arteries and into the lungs. In the lungs, carbon dioxide is released from the blood and oxygen is picked up.

  Pulmonary veins bring the oxygenated blood back to the heart and into the left atrium. From the left atrium the blood moves to the left ventricle which pumps it out to the body through the aorta. On both sides, the lower ventricles are thicker and stronger than the upper atria. The muscle wall surrounding the left ventricle is thicker and stronger than the wall surrounding the right ventricle because the left ventricle needs to exert enough force to pump the blood through the body. The right ventricle only needs to pump the blood as far as the lungs, which does not require as much contractile force.

  Valves in the heart maintain the flow of blood by opening and closing in one direction only. Blood can move only forward through the heart, and is prevented from flowing backward by the valves. Such movement of the blood is called unidirectional flow. There are four valves of the heart:

  The two atrioventricular (AV) valves ensure blood flows from the atria to the ventricles, and not the other way. The AV valve on the right side of the heart is called the tricuspid valve, and the one on the left of the heart is called the mitral, or bicuspid valve.

  The two semilunar (SL) valves are present in the arteries leaving the heart, and they prevent blood flowing back from the arteries into the ventricles. The SL valve on the right side of the heart is called the pulmonary valve, because it is leads to the pulmonary arteries, and the SL valve on the left is called aortic valve because it leads to the aorta. The valves of the heart are shown in Figure below.

  Figure 22.4

  The direction of blood flow through the heart.

  The Heartbeat

  The heart is a meshwork of cardiac muscle cells that are interconnected by little channels called gap junctions. This interconnection allows the electrical stimulation of one cell to spread quickly to its neighboring cells. Cardiac muscle is self-exciting. This is in contrast to skeletal muscle, which needs nervous stimulation to contract. The heart's rhythmic contractions occur spontaneously, although the frequency of the contractions, called the heart rate, can be changed by nervous or hormonal signals such as exercise or the perception of danger.

  Control of the Heartbeat

  The rhythmic sequence of contractions of the heart is coordinated by two small groups of cardiac muscle cells called the sinoatrial (SA) and atrioventricular (AV) nodes. The sinoatrial node (SA node), often known as the "cardiac pacemaker", is found in the upper wall of the right atrium and is responsible for the wave of electrical stimulation that starts atrial contraction by creating an action potential. The action potential causes the cardiac cells to contract. This wave of contraction then spreads across the cells of the atrium, reaching the atrioventricular node (AV node) which is found in the lower right atrium, shown in Figure below. The AV node conducts the electrical impulses that come from the SA node through the atria to the ventricles. The impulse is delayed there before being conducted through special bundles of heart muscle cells called the bundle of His and the Purkinje fibers, which leads to a contraction of the ventricles. This delay allows for the ventricles to fill with blood before the ventricles contract. Heartbeat is also controlled by nerve messages originating from the autonomic nervous system.

  There are important physiological differences between cardiac cells found in the nodes and cardiac cells found in the ventricles. Differences in ion channels and mechanisms of polarization give rise to unique properties of SA node cells, most importantly the spontaneous depolarizations necessary for the SA node's pacemaker activity.

  The Bundle of His is a collection of heart muscle cells (fibers) specialized for electrical conduction that transmits the electrical impulses from the AV node. The bundle of His branches into Purkinje fibers. Purkinje fibers, shown in Figure below, are specialized cardiac muscle cells that conduct action potentials into the ventricles, causing the cardiac muscle of the ventricles to contract in a controlled way.

  Figure 22.5

  Schematic representation of the atrioventricular Bundle of His. The SA node is blue, and the AV node is red and visible in the right atrium. The AV node forms the Bundle of His. Sometimes the left and right Bundles of His are called Purkinje fibers.

  Figure 22.6

  The larger round cells on the right are Purkinje fibers. Because of their specializations to rapidly conduct impulses (numerous sodium ion channels and mitochondria, fewer myofibrils than the surrounding muscle tissue), Purkinje fibers take up stain differently than the surrounding muscles cells, and on a slide, they often appear lighter and larger than their neighbors.

  The heartbeat is made up of two parts; muscle contraction and relaxation. Systole is the contraction of the heart chambers, which drives blood out of the chambers. Diastole is the period of time when the heart relaxes after contraction. All four chambers of the heart undergo systole and diastole in a timed fashion so that blood is moved forward through the cardiovascular system. For example, ventricular systole is the point at which the ventricles are contracting, and atrial systole is the point at which the atria are contracting. Likewise, ventricular diastole is the period during which the ventricles are relaxing, while atrial diastole is the period during which the atria are relaxing. In general, when referring to systole and diastole, the chambers being referred to are the ventricles, which is shown in Figure below.

  Figure 22.7

  When the atria contract, the blood gets pushed into the ventricles which are in d
iastole. When the ventricles contract (ventricular systole), the blood gets pushed out of the heart.

  Heart Sounds

  In healthy adults, there are two normal heart sounds often described as a "lub" and a "dub" that occur with each heart beat (lub-dub, lub-dub). In addition to these normal sounds, a variety of other sounds may be heard including heart murmurs or clicks. A medical practitioner uses a stethoscope to listen for these sounds, which gives him or her important information about the condition of the heart.

  The sound of the heart valves shutting causes the heart sounds, or a heartbeat. The closing of the mitral and tricuspid valves (known together as the atrioventricular valves) at the beginning of ventricular systole cause the first part of the "lub-dub" sound made by the heart as it beats. The second part of the "lub-dub" is caused by the closure of the aortic and pulmonic valves at the end of ventricular systole. As the left ventricle empties, its pressure falls below the pressure in the aorta, and the aortic valve closes. Similarly, as the pressure in the right ventricle falls below the pressure in the pulmonary artery, the pulmonic valve closes.

  Blood Vessels

  The blood vessels are part of the cardiovascular system and function to transport blood throughout the body. The two most important types are arteries and veins. Arteries carry blood away from the heart, while veins return blood to the heart.

  There are various kinds of blood vessels, the main types are:

  Arteries are the large, muscular vessels that carry blood away from the heart.

  An arteriole is a small diameter blood vessel that extends and branches out from an artery and leads to capillaries.

  Veins are vessels that carry blood toward the heart. The majority of veins in the body carry low-oxygen blood from the tissues back to the heart.

  A venule is a small vessel that allows deoxygenated blood to return from the capillaries to veins.

  Capillaries are the smallest of the body's blood vessels, that connect arterioles and venules, and are important for the interchange of gases and other substances between blood and body cells.

  The blood vessels have a similar basic structure. The endothelium is a thin layer of cells that creates a smooth lining on the inside surface of blood vessels. Endothelial tissue is a specialized type of epithelium, one of the four types of tissue found in the body. Endothelial cells have an important structural role in blood vessels; they line the entire circulatory system, from the heart to the smallest capillary. Around the endothelium there is a layer of smooth muscle, which is well developed in arteries. Finally, there is a further layer of connective tissue that surrounds the smooth muscle. This connective tissue, which is mostly made up of collagen, contains nerves that supply the smooth muscular layer. The connective tissue surrounding larger vessels also contains capillaries to bring nutrients to the tissue. Capillaries, the smallest blood vessels, are made up of a single layer of endothelium and a small amount of connective tissue.

  Figure 22.8

  The structure of an artery wall.

  Arteries and Arterioles

  The arteries carry blood away from the heart. As shown in Figure above, arteries have thick walls that have three major layers; an inner endothelial layer, a middle layer of smooth muscle, and an outer layer of stretchy connective tissue (mostly collagen). The elastic qualities of artery walls allow them to carry pressurized blood from the heart while maintaining blood pressure.

  The aorta is the largest artery in the body. It receives blood directly from the left ventricle of the heart through the aortic valve. The aorta branches, into smaller arteries and these arteries branch in turn, becoming smaller in diameter, down to arterioles. The arterioles supply the capillaries that carry nutrients to the body’s cells and tissues. The aorta is an elastic artery. When the left ventricle contracts to force blood into the aorta, it expands. This stretching gives the potential energy that will help maintain blood pressure during diastole when the aorta contracts passively.

  An arteriole is a small-diameter blood vessel that branches out from an artery and leads to capillaries. Arterioles have thin muscular walls, composed of one or two layers of smooth muscle, and are the primary site of vascular resistance. Vascular resistance is the resistance to flow that blood must overcome to be pumped through your circulatory system. Increasing vascular resistance is one way your body can increase blood pressure.

  Veins and Venuoles

  Figure 22.9

  Internal structure of a vein.

  Veins return deoxygenated blood to the heart. The thick, outer layer of a vein is made up of collagen-containing connective tissue, shown in Figure above. The connective tissue is wrapped around bands of smooth muscle while the interior is lined with endothelium. Most veins have one-way flaps called valves, shown in in Figure below, that prevent blood from flowing backward and pooling in the legs, feet, arms or hands due to the pull of gravity. The location of veins can vary from person to person.

  Figure 22.10

  Valves found in veins prevent the blood from flowing backward and pooling in the lowest parts of the body, such as the legs and feet.

  A venule is a small blood vessel that allows deoxygenated blood to return from the capillary beds to the larger blood vessels called veins. Venules have three layers: an inner endothelium composed of squamous epithelial cells that act as a membrane, a middle layer of muscle and elastic tissue, and an outer layer of fibrous connective tissue. The middle layer is poorly developed so that venules have thinner walls than arterioles.

  Capillaries

  Capillaries are the smallest of a body's blood vessels, measuring 5-10 μm in diameter. Their size is shown in relation to body cells in Figure below. Capillaries connect arterioles and venules, and they are important for the exchange of oxygen, carbon dioxide, and other substances between blood and body cells.

  Figure 22.11

  The structure of capillaries. Note their size in comparison to the cells around them.

  The walls of capillaries are made of only a single layer of endothelial cells. This layer is so thin that molecules such as oxygen, water and lipids can pass through them by diffusion and enter the body tissues. Waste products such as carbon dioxide and urea can diffuse back into the blood to be carried away for removal from the body. Capillaries are so small the blood cells need to pass through it in a single file line. A capillary bed is the network of capillaries supplying an organ. The more metabolically active a tissue or organ is, the more capillaries it needs to get nutrients and oxygen.

  Blood vessels are roughly grouped as arterial and venous. This grouping is determined by whether the blood in the vessel is flowing away from (arterial) or toward (venous) the heart. In general the term arterial blood is used to describe blood high in oxygen, although the pulmonary arteries carry deoxygenated blood and blood flowing in the pulmonary vein is rich in oxygen.

  Roles of Blood Vessels

  Blood vessels are not involved in regulating the transport of blood, the endocrine and nervous systems do that. However, arteries and veins can regulate their inner diameter by contraction of the smooth muscle layer. This widening or narrowing of the blood vessels changes the blood flow to the organs of the body. This process is controlled by the autonomic nervous system; it is not controlled consciously.

  Vasodilation is a process by which blood vessels in the body become wider due to the relaxation of the smooth muscle in the vessel wall. This reduces blood pressure since there is more room for the blood to move through the vessel. Endothelium of blood vessels uses nitric oxide to signal the surrounding smooth muscle to relax, which dilates the artery and increasing blood flow. Nitric dioxide is a vasodilator.

  Vasoconstriction is the constriction of blood vessels (narrowing, becoming smaller in cross-sectional area) by contracting the vascular smooth muscle in the vessel walls. Vasoconstriction is controlled by substances such as some hormones and neurotransmitters, which are called vasoconstrictors. For example, the “fight or flight” hormone epinephrine is a vasoconstricto
r that is released by the adrenal glands.

  Permeability of the endothelium is important for the release of nutrients to the tissue. Permeability is the ability of a membrane to allow certain molecules and ions to pass through it by diffusion. Permeability of the endothelium increases during an immune response, which allows white blood cells and other substances to get to the site of injury or irritation.

  Oxygen, which is bound to hemoglobin in red blood cells for transport through the body, is the most critical nutrient carried by the blood. In all arteries apart from the pulmonary artery, hemoglobin is highly saturated (95-100%) with oxygen. In all veins apart from the pulmonary vein, the hemoglobin is desaturated at about 70%. (The values are reversed in the pulmonary circulation.)

  Blood Vessels and Blood Pressure

  Blood pressure refers to the force exerted by circulating blood on the walls of blood vessels. The pressure of the circulating blood gradually decreases as blood moves from the arteries, arterioles, capillaries, and veins. The term "blood pressure" generally refers to arterial pressure, which is the pressure in the larger arteries that take blood away from the heart. Arterial pressure results from the force that is applied to blood by the contracting heart, where the blood “presses” against the walls of the arteries.